1. Field of the Invention
The present invention relates to a transmission output control apparatus and its method and a transmitter using the same and, more particularly, to a transmission output control technique for controlling transmission power in a transmitter.
2. Description of the Related Art
FIG. 1 shows an example of a conventional transmission output control apparatus. In FIG. 1, a mixer 2 and a synthesizer 5 transform the output of a modulator 1 into an RF signal and supply it to an amplifier 4 via a variable attenuator 3. The variable attenuator and the amplifier 4 amplify the RF signal to a predetermined transmission output and output it. A directional coupler 8 extracts part of the output of the amplifier 4 and supplies it to a detector 8. The detector 8 detects the extracted signal. A comparator 9 derives a differential signal between a detection voltage and a first reference voltage 12. Then, a low-pass filter (LPF) 20 and an error amplifier 6 suppress the variation in the differential signal as well as amplifying it. The error amplifier 6 supplies the output to the variable attenuator 3 as an attenuation amount control voltage. The variable attenuator 3 changes its gain based on the attenuation amount control voltage from the error amplifier 6. In this manner, the transmission output control apparatus in FIG. 1 controls such that the transmitter output keeps a fixed level at all times.
FIG. 2A shows the output voltage of the modulator in FIG. 1. The instantaneous level variation in the output voltage is large. Therefore, even if the LPF 20 performs smoothing of the output of the comparator 9, it is difficult to sufficiently suppress the variation. Because of this, as shown in FIG. 2B, the transmitter output level follows the envelope of the modulator output to a certain extent.
To prevent this, it is conceivable to increase the time constant of the LPF 20. However, if the time constant of the LPF 20 increases, troubles will occur when it is necessary to change the transmitter output level. In other words, if changing the transmitter output level by changing the first reference voltage, the LPF 20 suppresses the change in output of the comparator 9 accompanying the change in the first reference voltage. Because of this, it takes time for the transmitter output level to reach a predetermined transmission output level after the change. Cases where it is necessary to change the transmitter output level include a case of starting the apparatus and a case of changing the frequency. Further, also when another apparatus is performing an Automatic Transmit Power Control (ATPC), the transmitter changes the output level.
Japanese Patent Application Laid-Open No. HEI 8-97734 (1996) (document 1) disclose examples of techniques relating to such a transmission power control circuit. The document 1 discloses a transmission power control circuit of a transmitter using both the AM modulation wave and the FM modulation wave. The transmission power control circuit changes the time constant of an LPF according to the type of modulation wave. However, this technique does not sufficiently solve the problem caused by an instantaneous level fluctuation in the transmission signal.
Japanese Patent Application Laid-Open No. HEI 8-102767 (1996) (document 2) discloses a transmission power control circuit in which the envelope feedback loop in the transmitter controls a transmitter output level such that the baseband modulation signal becomes the same as the envelope of the modulated signal when gain fluctuation of a high frequency power amplifier etc. occurs. This transmission power control circuit outputs modulated waves at a fixed power regardless of the gain fluctuation of the power amplifier. However, it does not sufficiently solve the problem about the instantaneous level change in the transmission signal.